1. Field of the Invention
The present invention relates to a droplet ejection device comprising a pressure chamber, a nozzle orifice in fluid connection with the pressure chamber, and an actuator system for generating a pressure wave in the liquid in the pressure chamber.
2. Description of Background Art
Droplet ejection devices are used, for example, in ink jet printers for ejecting ink droplets onto a recording medium. The actuator system may, for example, comprise a piezoelectric actuator that, when energized, performs a contraction stroke followed by an expansion stroke so as to generate an acoustic field primarily in an ejection liquid (e.g. ink present in the pressure chamber and resulting in a droplet of the ejection liquid (e.g. an ink droplet) being ejected from the nozzle orifice.
It is a disadvantage of droplet ejection devices that air bubbles can easily enter into the pressure chamber via the nozzle orifice. In particular, when after droplet ejection, the liquid-air interface (e.g. the ink meniscus) moves back into the interior of the droplet ejection device due to a residual pressure wave that propagates through the liquid (e.g. ink). If the liquid-air interface moves relatively far into the interior of the droplet ejection device, the surface energy of the liquid-air interface may cause formation of air bubbles in the liquid. The presence of air-bubbles may negatively influence the jetting stability and is therefore an undesired phenomenon. Maintenance actions (e.g. purging) may be required to remove air bubbles before the jetting process can be reliably resumed.
In order to avoid entrapped air, a nozzle orifice design comprising a gradual geometric transition from the nozzle orifice towards the pressure chamber may be used. Such geometry also provides smooth guidance of a liquid from the pressure chamber to the nozzle orifice, optionally via a feed-through channel arranged as a part of the pressure chamber and extending towards the nozzle orifice, From a manufacturing point of view, such nozzle orifice design is less preferred, because a large number of processing steps is involved in manufacturing such nozzle orifices. Moreover, the allowable geometrical tolerances of such nozzle orifice designs in order to meet the jetting requirements (e.g. jetting angle and jetting stability) are small, which are difficult to obtain with such a multi-step processing.
From the manufacturing point of view, straight nozzle orifices having a first dimension S1 (e.g. for a cylindrical nozzle, a first diameter d1) connected to a straight feed-through channel having a second dimension S2 (e.g. for a cylindrical feed-through channel, a second diameter d2), wherein S2 is larger than S1 (d2>d1), is preferred. In such a configuration, the geometrical transition between the nozzle orifice and the feed-through channel comprises a discrete step. Manufacturing such nozzle orifice and feed-through channel designs comprises less process steps and the geometrical tolerance on the connection between the nozzle orifice and the feed-through channel is less critical.
A disadvantage of droplet ejection devices having straight nozzle orifices connected to a straight feed-through channel is that air bubbles that have entered the pressure chamber via the nozzle orifice may be difficult to be removed. Without wanting to be bound to any theory, this may be caused by the presence of dead volumes in a feed-through channel that is connected to a straight nozzle. If the entered air bubbles end up in said dead volumes, they may be more or less permanently entrapped or at least difficult to be removed.
U.S. Application Publication No 2008/0088669 A1 discloses a nozzle plate comprising nozzle orifices having a first cylindrical columnar part and a second cylindrical columnar part, the first columnar part having a larger diameter than the second columnar part. The second columnar part is arranged for discharging droplets. A droplet guidance part having a cylindrical columnar shape is coaxially arranged in the first columnar part and supported by a first support.
The first and the second columnar parts are manufactured separately from the droplet guidance part and assembled afterwards. The first support supporting the droplet guidance part is fixed to the first columnar part.
A disadvantage of the nozzle plate design disclosed in U.S. Application Publication No. 200810088669 A1 is that the droplet guidance part is only supported at a first end of the droplet guidance part, the first end being opposite to a second end of the droplet guidance part, which second end faces the nozzle orifice. The droplet guidance part therefore has a free end (i.e. unsupported) facing the nozzle orifice, i.e. the second end of the droplet guidance part. In operation, the free end of the droplet guidance part may freely move (e.g. vibrate), which may cause jet instabilities. Due to said free movement, sucked in air bubbles may be broken down into small air bubbles, which are difficult to be removed.
Another disadvantage of the nozzle plate design disclosed in U.S. Application Publication No. 2008/0088669 A1 is that the first and the second columnar parts are manufactured separately from the droplet guidance part and assembled afterwards, which is a rather complex manufacturing process comprising alignment steps that may introduce alignment errors.